The concept of all-wheel steering is well known. Dozens of all-wheel steering systems are disclosed in U.S. and foreign patents. Some known all-wheel steering systems are purely mechanical, meaning that the rear wheel angle is determined by gears and gear ratios manually selected by the vehicle operator with the shift of a lever. Other all-wheel steering systems provide rear wheel steering independent of the front wheel angle where the operator steers the rear wheels independently of the front wheels, the rear wheels being steered with a separate steering wheel, joy stick or potentiometer.
Automatic methods of all-wheel steering are also known. In these systems, a microcomputer will control the rear wheel angle in accordance with various physical relationships that are functions of various parameters. For example, it is known to control the rear wheel angle in accordance with the front wheel-to-rear wheel steering angle ratio. It is also known to control the rear wheel angle indirectly by controlling the rear wheel speed so that the desired rear wheel speed is nearly equal to the actual front wheel speed. Other systems control the rear wheel angle after solving a series of complex equations that are functions of the steering angle, vehicle speed and other parameters. Finally, in some all-wheel steering systems, the rear wheel angle is a function of the speed of rotation of the steering wheel.
Our concept of all-wheel steering has been applied to larger vehicles such as trucks and aircraft rescue and fire fighting equipment (ARFF). These vehicles must be able to maneuver into small spaces in short periods of time. All-wheel steering provides great benefits in this regard because it provides a smaller minimum turning radius. However, it would be beneficial to provide an all-wheel steering system for these large vehicles that would keep the rear wheels in the straight-ahead position at highway speeds because larger vehicles such as motorhomes and ARFF equipment are often operated by inexperienced drivers. Specifically, motorhomes are often rented and driven by tourists; even ARFF trucks may be operated by volunteer fire fighters who may lack the requisite experience to operate such large equipment. Thus, all-wheel steering at high speeds can pose serious dangers for larger, heavier vehicles and this situation if often compounded when an inexperienced driver is behind the wheel.
The prior art teaches all-wheel steering systems with four principal steering modes: front wheel only, crab, coordinated and independent rear wheel. In front wheel only steering, the steering of the vehicle is dependent upon the front wheel position only and the rear wheels maintain a straight-ahead position, or 0.degree. steering angle. Front wheel only steering is appropriate at both low and high speeds.
In crab steering, the front wheels and rear wheels are steered in the same direction. Crab steering improves the vehicle's stability at high speeds allowing the vehicle to change lanes and avoid obstacles with less vehicle yaw and improved traction. Crab steering lowers the lateral g-forces by increasing the turning radius for a given front wheel angle thereby lowering the chances of roll-overs for vehicles with relatively high centers of gravity, such as aircraft, rescue and fire fighting vehicles (ARFF). Because the rear of the vehicle is also steered to the side by the rear wheels, the increased turning radius is not detrimental to obstacle avoidance. Crab steering is also beneficial to passenger cars because it allows the driver to change lanes or avoid obstacles quickly and safely. Crab steering is effective at moderate to high speeds.
In coordinated steering, the front wheels and rear wheels are turned in generally opposite directions. Coordinated steering is safely employed for low speed maneuvers only. Coordinated steering improves the vehicle's maneuverability by reducing the minimum turning radius. This increased maneuverability is especially beneficial to vehicles with long wheel bases such as heavy duty trucks and ARFF equipment, but is also greatly appreciated by the drivers of passenger cars. The reduced minimum turning radius makes it easier for the drivers of passenger cars as well as trucks to maneuver and park in congested city traffic.
Further, in independent rear wheel steering, the rear wheels are controlled independently of the front wheels. Rear wheel steering requires an additional and separate steering mechanism for the rear wheels which is separate and apart from the conventional steering wheel used to control the front wheels.
The present invention makes several contributions to the art of all-wheel steering. First, the present invention provides two improved means for calculating or selecting the desired rear wheel angle based upon the front wheel angle. A first means of calculating the desired rear wheel angle employs a relational curve to be used which varies with speed and features a programmable "dead band" that allows a certain amount of front wheel movement during which the rear wheels will remain in the straight-ahead or 0.degree. position. The dead band can be fixed or vary depending upon the specific design. In one embodiment, the dead band is fixed at lower speeds and increases at higher speeds to ensure that the rear wheels remain in a straight ahead position at higher speeds. In another embodiment designed for large vehicles such as trucks with substantial rear overhang, the dead band is set at a large value at low speeds because the rear ends of trucks with long overhangs can engage other vehicles or buildings during low speed maneuvers if the rear wheels are permitted to turn.
A second improved means for calculating the desired rear wheel angle employs a relational curve that may be used for straight-ahead, coordinated and crab modes of steering. The operator or programmer may select from a variety of relational curves or polynomial equations in order to tailor the front to rear relational curves to the vehicle. Further, the driver may select from a variety of relational curves so that different relational curves may be provided for different driving conditions.
Further, the present invention alleviates the steering problem known as off-tracking that is most commonly associated with trucks and trailers or tractor-trailer rigs. Off-tracking occurs when the driver turns causing the side of the trailer or vehicle to move laterally in the direction of the turn. The amount of off-tracking is proportional to the angle of the turn and the vehicle wheel base and occurs to a certain extent for all turns greater than 0.degree.. If the driver doesn't take the turn at an angle that is wide enough, the trailer or vehicle can engage the curb or an object located on the inside of the turn, such as a signal light. The solution to this problem is to provide all wheel steering that can be used with trucks or trailers to allow them to make sharp turns in the coordinated mode. In the coordinated mode, the rear truck or trailer wheels will steer in a direction opposite of the turning direction or the direction of the front wheels thereby steering the vehicle around obstacles located on a street corner. In effect, the rear of the truck or trailer can more accurately track the path of the front of the vehicle thereby alleviating the off-tracking problem.
In one preferred embodiment, a delay in turning the rear wheels of the truck or trailer based on distanced travelled enhances the tracking of the rig through a turn because the delay enables the rear of the truck or trailer to more accurately follow the path of the front of the vehicle without the rear travelling outside of the front of the truck's path at the start of the turn. Crab steering enhances the obstacle avoidance and lane-changing ability of tractor-trailer rigs and is useful when backing up tractor-trailer rigs because it makes them behave like straight trucks.
Finally, the present invention allows the driver to manually select from front wheel only, crab, coordinated and independent rear wheel steering modes as well as a variety of all-wheel steering modes including modes with increased rear wheel cramp angles for increased maneuverability in tight spots.